Integrated circuit technology relies on transistors to formulate vast arrays of functional circuits. The complexity of these circuits requires the use of an ever increasing number of linked transistors. As the number of transistors required increases, the surface area that can be dedicated to a single transistor dwindles. It is desirable then, to construct transistors which occupy less surface area on the silicon chip/die.
In one example, integrated circuit technology uses transistors conjunctively with Boolean algebra to create a myriad of digital circuits, also referred to as logic circuits. In a typical arrangement, transistors are combined to switch or alternate an output voltage between just two significant voltage levels, labeled logic 0 and logic 1. In an alternate example, integrated circuit technology similarly uses transistors combined with capacitors to form memory cells. Here, the data is stored in electronic form as a charge on the capacitor. The charge, or absence of charge, on the capacitor translates to either a logic 1 or 0. Most logic systems use positive logic, in which logic 0 is represented by zero volts, or a low voltage, e.g., below 0.5 V; and logic 1 is represented by a higher voltage.
Integrated circuits, including transistors, are typically formed from either bulk silicon starting material, silicon on insulator (SOI) starting material, or SOI material that is formed from a bulk semiconductor starting material during processing. The SOI complementary metal oxide semiconductor (CMOS) technology, however, has been viewed as a likely successor to conventional bulk technology since it provides a prospect of greater circuit performance. This increase in performance results from the lower parasitic junction capacitances and the improved transistor characteristics and tolerances. Basic to the feature of isolating the active silicon layer from the substrate by an intervening insulator layer is the so called "floating body" effect on device characteristics. Since the bodies of individual devices are not in direct electrical contact to the conducting substrate, their electrical potential can vary with time depending on leakage currents and parasitic capacitive coupling to other electrodes. Such an effect is clearly undesirable and represents a major stumbling block to the introduction of SOI as a viable product technology.
One approach to handle the uncertain body potential is to include margins within the circuit design to allow for the floating body effect. While this requires no technology action, it diminishes the performance benefits of SOI. Another approach is to minimize the floating body effect by providing an enhanced leakage path to the device body from the device source. This is a partial solution since it merely limits the amount by which the body potential may vary relative to the source and does not allow the voltage to be set at any particular optimum value. Further, it necessarily creates an electrically asymmetric device which limits its acceptability. Other techniques include providing a separate conducting contact to the device bodies. To date, however, the methods proposed have proven cumbersome and come at the price of decreased device density or a compromise in the lower parasitic junction capacitance that motivates the use of SOI.
Thus what is needed is an improved method and structure for implementing SOI transistors, or devices, which provide a predictable electrical potential in the body of the device without sacrificing the benefits attained from using the SOI structure. Any improved method and structure should also conserve surface space on the semiconductor die and maximize device density.